Project Summary Obesity is a significant and growing public health problem. It results from loss of energy homeostasis when increases in caloric intake are not balanced by elevated metabolic activity. Brown adipose tissue (BAT) is a potential anti-obesity organ because it is specialized to dissipate energy as heat. Circannual hibernators including ground squirrels naturally challenge conventional views of energy homeostasis by cycling between dramatic extremes of body fat storage and utilization. These animals undergo hypertrophy of BAT each year as they prepare for winter hibernation, and depend on tightly-regulated periods of non-shivering thermogenesis throughout winter hibernation. After spring emergence, BAT nearly disappears but it is re- established each fall in preparation for the next hibernation season. These temporal and functional segregations provide an extraordinary opportunity to examine and define regulatory mechanisms controlling BAT recruitment and activity. We hypothesize that hibernators employ typical mammalian mechanisms for increasing and maintaining BAT, as well as for modulating its activity. Importantly, because of the dynamics and demands of hibernation, hibernators also provide an exceptional, highly-elaborated model system in which to elucidate those mechanisms. The specific goals of the work proposed in this R21 application are to exploit an existing tissue bank, collected from a precisely-timed set of animals representing key points in the phenotypic transitions of a hibernator's year, to first: interrogate the proteome and second: the transcriptome, for qualitative and quantitative changes associated with BAT transitions. This characterization, enabled by unique advantages offered by the hibernating phenotype, will form a necessary foundation for future mechanistic studies of pathways that underlie the regulation of BAT recruitment and the reversible control of its activity, and ultimately to harness them as targets for obesity therapy.